P7- Studying the Universe

·         A sidereal day is the time taken for a STAR to return to the same position in the sky. it’s about 23 hours and 56 minutes

·         The Sun seems to move more slowly across the sky than distant stars - it takes 24hours to get to the same poisition in the sky, a whole 4 minutes longer

·         A solar day is the time taken for the Sun to appear at the same position in the sky. it takes 24 hours

·         Solar and sidereal days are different because the Earth orbits the sun as well as spinning on its axis. 

·         The Earths orbits the sun in the same direction as it spins- so the Earth needs to spin slightly more than 360* before the sun appears at the same position in the sky

·         The Moon seems to go more slowly than the Sun, taking about 25 hours. This is because the Moon orbits the earth in the same direction as the Earth is rotating

·         As the Earth moves around the sun the direction we face changes slightly each day. this means we see different stars 

·         The Moon doesn’t glow itself- it only reflects light from the sun. only the half facing the Sun is lit up, leaving the other half in shadow. As the Moon orbits the Earth, we see different amounts of the Moon’s dark and lit-up surfaces. You see a ‘full moon’ when the whole of the lit-up surface is facing the Earth, and a ‘new moon’ when the dark half faces us.

·         Eclipses happen when the light from the Sun is blocked. There are two main types of eclipses: lunar and solar…

·         LUNAR ECLIPSE=the Moon sometimes passes into the Earth’s shadow. The Earth blocks sunlight from the Moon, so almost no light is reflected from the Moon and it just seems to disappear. A total lunar eclipse is where no direct sunlight can reach the moon, but partial lunar eclipse are much more common

·         SOLAR ECLIPSE=The moon is just the right size and distance away that when it passes between the Sun and the Earth, it can block out the sun. From some parts of the Earth the Sun is completely blocked – a total solar eclipse. From many places on Earth only part of the Sun will be blocked – a partial solar eclipse.

·         Eclipses don’t happenvery often. The moon orbits the Earth at an angle to Earth’s orbit around the Sun. so most of the time the Sun, Moon and Earth don’t line up to cause eclipses.

·         The positions of stars are measured by angles seen from Earth. The sky appears to turn as the Earth spins- so astronomers picked two fixed positions to measure from:

·         THE POLE STAR= is a star that doesn’t seem to move because it’s almost directly above the North Pole

      THE CELESTIAL EQUATOR= is an imaginary plane running across the sky, extending out from the Earth’s equator

·         The two angles used to measure positions in the sky are:

·         Declination- celestial latitude, measured in degrees

·         Right Ascension- celestial longitude measured in degrees or time

·         Planets seem to move in complicated patterns. The closer to the Sun, the quicker the planet Even without a telescope, you can often see the ‘naked eye’ planets –Mercury, Venus, Mars, Jupiter and Saturn. Every so often, though, a planet seems to change direction –this is called retrograde motion. It only happens with the outer planets- Mars to NeptuneIt happens because both the planet and Earth are moving around the Sun- o we’re seeing the motion of the planet relative to Earth. Slower-moving plants further out ‘change direction’ less frequently.

·         Refraction- Waves change speed and Direction. The speed of a wave is affected by the density of the substance or medium it’s travelling in. So when a wave crosses a boundary between two substances, it changes speed.The change in speed and wavelength can cause the wave to change direction – this is called refraction.

·         If a light wave hits the boundary ‘face on’ it slows down but carries on in the same direction. It now has a shorter wavelength but the same frequency. But if a wave meets a different medium at an angle, part of the wave hits the boundary first and slows down, whole another part carriers on at the first, faster speed for a while. So the wave changed direction – it’s been REFRACTED.

·         Converging or convex lenses use refraction to focus light waves to form an image on an object. As a ray hits the surface of the lens, it slows down. This causes the light ray to bend towards the ‘normal’. When it hits the ‘glass to air’ boundary on the other side it speeds up and bends away from the normal.

·         The curvature of the lens means all the parallel rays hitting different parts of the lens are bent towards the same focal point, where an image is formed of whatever the light is coming from.

·         DIFFERENT WAVELENGTHS OF LIGHT REFRACT BY DIFFERENT AMOUNTS. So white light spreads out into its different colours as it enters a prism. 

·         A converging lens is convex – it gets fatter towards the middle. It causes rays of light to converge (come together) to a focus.

·         All lenses have principal axis, a line which passes straight through the middle of the lens. The focal point of a lens is where rays initially parallel to the principal axis meet ( All lenses have two focal points, one in front of the lens, and one behind the lens).

·         The focal length of a lens is just the distance between the middle of the lens and its focal point. Focal length is related to power. The more powerful the lens, the more strongly it converges parallel rays of light, so the shorter the focal length.

·         Power (D) =

·         A simple refracting optical telescope is made up of two convex lenses with different powers- an objective lens and a more powerful eye lens (or eyepiece). The objective lens collects the light and forms and image of it, the eyepiece magnifies this image so we can view it.

·         The lenses are aligned to have the same principal axis and are placed so that their focal points are in the same place.

·         The objective lens converges these parallel rays to form a real image between the two lenses. The eyepiece lens is much more powerful than the objective lens (its much more curved). It acts as a magnifying glass on the real image and makes a virtual image.

·         Magnification =

·         Most astronomical telescopes use a concave mirror instead of a convex objective ens. Concave mirrors are shiny on the inside of the curve. Parallel rays of light shining on a concave mirror reflect and converge.

·         Rays parallel to the mirror’s axis, e.g. those from a distant star, reflect and meet at the focal point. By putting a lens near the focal point of the mirror to act as an eyepiece, you can form a magnified image.

All waves (‘diffract’) at the edges when they pass through a gap or past an object. The amount of diffraction depends on the size of the gap relative to the wavelength. The narrower the gap, or the long the wavelength the more the wave spreads out. 

Some objects in the sky are so distant and faint, only a tiny amount of radiation from them reaches us. To collect enough of the radiation, you need to use a telescope with a huge objective lens. The diameter of the objective lens is called the aperture. The bigger the aperture, the more radiation can get into the telescope and the better the image formed. Making large lenses is difficult and expensive, whereas big mirrors are much easier to make accurately. This is one of the reason why many telescopes have a concave mirror instead of a lens.

· Because all waves can diffract, radiation entering a telescope spreads out at the edges of the aperture – causing a image to blur. The only way round this problem is to have an aperture that’s much wider than the wavelength of radiation. This way the radiation passes through the aperture and into your telescope with very little diffraction and you get a sharp image.

· A diffraction grating have very narrow slits – small enough to diffract light. When white light passes through the gaps, the different wavelengths of coloured light are all diffracted but by different amount. Astronomers can use these spectra to analyse the light coming from stars.

The parallax angle is half the angle moved against distant background stars over 6 months (at the opposite ends of the Earth’s orbit). The nearer an object is to you, the greater the anle.

·         This angle is often measured in arcseconds rather than degrees: 1as = 1”= 1/3600. Parallax is useful for calculating the distance to nearby stars. The smaller the parallax angle, the more distant the star is. Astronomers usually use a unit of distance called a parsec. A parsec (pc) is the distance to a star with a parallax angle of 1 arcsecond. Distances between stars are normally a few parsecs. You can calculate the distance to a star (in parsec): Distance= 

· Observed intensity depends on the distance to the star: the luminosity of a star depends on its size and temperature. The bigger and hotter it is, the more energy it gives out, so the brighter it is. As you move away from a star, it looks dimmer because the energy reaching you gets less. So the observed intensity of the light of a star as seen on Earth depends on its luminosity and how far away it is from Earth. 

Cepheid Variable: a group of stars, pulse in brightness, the get brighter and then dimmer over a period of several days. How quickly they pulse is directly linked to their luminosity. The greater the luminosity the longer the time between pulses. Astronomers can work out the distance to a Cepheid Variable by comparing the luminosity and the observed brightness.

·         Our sun is just one of the approximately  stars in the galaxy (this was worked out after a lot of peering through telescopes). The Milky Way is actually a spiral galaxy. But because we’re part of its disc, we see it edge on as a bright ***** in the sky. THE CURTIS-SHAPLEY DEBATE: In the 1920s there was a debate about the size and structure of the Universe. Shapley and Curtis argued about what these nebulae were and where they were.

·SHAPLEY’S ARGUMENT= Shapley believed the universe was just one gigantic galaxy about 100000 parsecs across. He reckoned our Sun and Solar system were far from the centre of the galaxy. He believed that nebulae were huge clouds of gas and dust. These clouds were relatively nearby and actually part of the Milky Way.

·CURTIS’ ARGUMENT= Curtis thought that the Universe was made up of many galaxies. He thought our galaxy was smaller than Shapley suggested- about 10 000 pc across, with the Sun at or very near the centre. The spiral nebulae were other very distant galaxies, completely separate from the Milky Way.

·Shapley was right that the Solar system is far from the centre of our galaxy, but Curtis was right that there are many galaxies in the Universe .Curtis was also right about spiral nebulae – Hubble used Cepheid Variable stars to show that they’re really far away.

·         Hubble helped solve the Curtis-Shapley debate with his observations of the Andromeda nebula. Hubble calculated the distance to the Andromeda nebula by working out the distance to the Cepheid variables within it, using the relationship between their brightness and pulse frequency. He studied other spiral nebulae and found a similar result- they were all too far away to be part of the Milky Way, and so must be separate spiral galaxies themselves.

·         When a galaxy is moving away from us the wavelength of the light from it changes – the light becomes redder. This is called red shift. By seeing how much the might has been red-shifted you can work out how quickly it is moving away. The greater the red shift, the greater the speed of recession.

·         Hubble: the more distant the galaxy, the faster it is moving away from us. This suggests that the whole universe is expanding from a single point.

·         The distance to a distant galaxy can be found from its recession velocity using Hubble’s law:

·         Speed of Recession = Hubble Constant x distance

·         (km/s)                                 (s-1)                  (km)

·         (km/s)                                 (km/s per Mpc)  (Mpc)

·         Hubble constant = 2 x  s-1 or 70km/s per Mpc 

·         Kinetic theory says that gases consist of very small particles. These particles are constantly moving in completely random directions. They constantly collide with each other and with the wall of their container. When they collide, they bounce off each other. The particles hardly take up any space. Most of the gas is empty space.

·         If you increase the temperature of something, you give its particles more kinetic energy- they more about more quickly or vibrate more. In the same way, if you cool a substance down, you’re reducing the kinetic energy of the particles.

·         The coldest that anything can ever get is -273C (0 K ) – this temperature is known as absolute zero. At absolute zero, atoms have as little kinetic energy as its possible to get. Absolute zero is the start of the Kelvin scale.

·         To convert from degrees Celsius to kelvins, just add 273. And to convert from kelvins to degrees Celsius, just subtract 273.

·         Anything that’s moving has kinetic energy. The temperature of a gas (in kelvins) is proportional to the average kinetic energy of its particles.

· As gas particles move about, they bang into each other. Gas particles have some mass, so when they collide with something, they exert a force on it. In a sealed container, gas particles smash against the container’s walls – creating an outward pressure.

· If you put the same amount of gas in a bigger container, the pressure will decrease there’ll be fewer collisions between the gas particles and the container’s walls. When the volume’s reduced, the particles get more squashed up and so they hit the walls more often, hence the pressure increases. At constant temp : pressure x volume = constant

· The pressure of a gas depends on how fast the particles are moving and how often they hit the walls. If you heat a gas, the particles move faster and have more kinetic energy. This increase in kinetic energy means the particles hit the container walls harder and more often, creating more pressure. At constant volume : pressure / temperature (in K) = constant

·Increasing the temperature increase the volume. If a gas stays at a constant pressure, then heating it up increases it volume – the molecules are further apart so collisions happen less frequently, but with more force.

·         At constant pressure: volume / temperature (in K) = constant

In the early 20^th century, Einstein realised that mass could be converted to energy. It was suggested that hydrogen is turned into helium inside the Sun, and that when this happens some mass gets ‘lost’. Perhaps the missing mass was being changed into energy – and powering the sun.

·         Two nuclei can combine (fuse) to create a larger nucleus, in stars hydrogen nuclei fuse together to make helium nuclei. Energy is released when lighter nuclei fuse to make heavier nuclei up to the size of an iron nucleus. Nuclei can only fuse like this if they are brought close together, at high temperatures and high pressures.

·         Albert Einstein reckoned that mass is a form of energy. So, mass can be converted into other forms of energy. When nuclei undergo nuclear fusion, they lose mass and energy is released

·         E = m

·         E= amount of energy released

·         M= amount of mass lost

·         C= speed of light in a vacuum = 300 000km/s 

Hot objects always emit more of one frequency than any other. This wavelength is called the peak frequency. The peak frequency emitted by an object depends on its temperature. The higher the temperature, the more energy the photons radiated will have, and so the higher the peak frequency. The luminosity of brightness also depends on temperature. We can tell how hot a star is by looking at its colour: red = has a low frequency= a cool star. Blue= has a high frequency= hot star.

· Electrons can only be in certain energy levels. Electrons move between energy levels if they gain or lose energy. ABSORPTION SPECTRA = at high temperatures, electrons become excited and jump into higher energy levels by absorbing radiation. Because there are only certain energy levels an electron can occupy, electrons absorb a particular frequency of radiation to get to a higher energy level. You can ‘see’ this if a continuous spectrum of visible light shines through a gas – the electron in the gas atoms absorb certain frequencies of the light, making gaps in the continuous spectrum. These gaps appear as dark lines,

·  EMISSION SPECTRA= electrons are unstable in the higher energy levels so they tend to fall from higher to lower levels, losing energy by emitting radiation of a particular frequency. This gives a series of bright lines formed by the emitted frequencies.Energy levels in atoms are different for each element, so each element has its own line spectrum 

·Stars are born in a cloud of dust and gas (which is mostly hydrogen and helium). Gravity causes the denser regions of the cloud to contract very slowly into clumps. When these clumps get dense enough, the cloud breaks up into protostars. Protostars continue to collapse under gravity – reducing in volume. This makes the particles more squashed up, increasing pressure and temperature. Eventually the temperature at the centre of the protostar reaches a few million degrees and hydrogen nuclei start to fuse together to form helium. This releases an enormous amount of energy and creates pressure to stop the gravitational collapse. The star has now reached the MAIN SEQUENCE STAGE.

·Fusion happens in the core of a star. The closer to the centre of the star, the hotter that bit will be. CORE= most of the fusion in a star takes place in the centre. The pressure from the weight of the rest of the star makes the core hotter and denser than the rest of the star. So the nuclei in the core are close enough to fuse. SURAFCE (photosphere) =the outer region of the star, from where the energy is radiated into space. Energy is released from fusion in the core is transported by photons of radiation and convection currents to the surface of the star.

·A star stops being in the main sequence stage when it runs out of hydrogen in the core. It then swells up to become a red giant or supergiant star. In the process, the photosphere cools down. The more massive a star is the hotter the core is. The hotter the core, the heavier the nuclei.

·         Most telescopes are now computer controlled – and this has many advantages:

1.     Instead of an astronomer having to always be there, they now just program the telescope to track an object in the sky.

2.     Computer control is also very useful when astronomers are using a telescope to do a survey – scanning across large areas of the sky in search of particular objects. For this telescopes need to be constantly repositioned to look at different areas of the sky. Thanks to computer control, this can just be programmed in too.

3.     Computer control allows telescopes to be positioned more precisely

4.     Astronomers like putting telescopes in remote locations. Before this had mean that astronomers would have to travel a long way and spend money and time on travelling but now they don’t have to

5.     Computers are also used to record and process data

6.     Without computer control we wouldn’t be able to have space telescopes

· Astronomers need accurate measurements to be able to understand what’s going on in space, but our atmosphere can mess up the results. Our atmosphere only lets certain wavelengths of electromagnetic radiation through and blocks all the other.

·Visible lights can be badly affected. Light gets refracted by water in the atmosphere, which blurs the images. Sites for astronomical observatories on earth are picked very carefully to try and minimise all these problems.·The first space telescope (Hubble) was launched by NASA in 1990. It can see objects that are about a billion times fainter.

·Getting a telescope safely into space is hard. And when things go wrong, it’s difficult to get the repair men out. Hubble’s first pictures were all fuzzy, because the mirror was the wrong shape. Most astronomy is still done using Earth-based telescopes as they’re a lot cheaper and easier to build and maintain. Astronomers have also developed good techniques to remove the effects of the atmosphere from their measurements so the images are clearer

·Space programmes are projects to send things like people, probes and telescopes into space. They’re really expensive.Governments have to balance paying these sums with other costly priorities like defence, healthcare and coping with nature disasters. The funding for space programmes is never guaranteed- there can be cut-backs at any time. 

·         Whether it’s building a new telescope on Earth or sending people into space, many science projects are too expensive for one country to do alone. These ‘big science’ projects are only possible if several countries cooperate and share the costs and resources.

·         By working together, you can get the best people and the best facilities for the job

·         Optical (visible light) observatories are often put in remote locations. The idea is to avoid man-made light pollution as well as dust and other particles affecting the observatories.

·         Astronomers want as little atmosphere between the observatory and telescope as possible to minimise the distorting and blurring effect it has. So observatories are often built at high elevation where the atmosphere is thinner.

·         Water in the atmosphere can cause problems by refracting light – so a dry location with low atmosphere pollution is good for a telescope.

·         Clouds block a telescopes view of the sky, so they’re built in places that have loads of cloudless night. Other factors need to be taken into account: Cost,  Access, Environment and Social